FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 1. Wood Structure | 5. Finishing Wood Structures | 1-64. General
9/8/98
SECTION 5. FINISHING WOOD STRUCTURES
AC 43.13-1B
1-64. GENERAL. Any repair to spars, ribs,
skin surfaces, or other structural parts of the
airframe involves finishing as the final step of
the job. The surface finish is the final line of
defense to prevent the destructive effects of
moisture entry into the structure. The time and
effort spent during the preparatory phase of the
refinishing process will be reflected in the ap-
pearance and longevity of the finished surface.
Adherence to the instructions issued by the
finish manufacturer is necessary to obtain the
appearance desired and protective characteris-
tics for the product used. The primary objec-
tive of interior finishes is to afford protection
of the wood against serious change in moisture
content when exposed to damp air or to water
that gains entrance to closed spaces by conden-
sation or by penetration of rain, mist, or fog.
Coatings, on contact areas between wood and
metal protect the metal against corrosion from
moisture in the wood. The primary objectives
of the exterior finish are to protect the wood
against weathering, provide a suitable appear-
ance, and present a smooth surface in flight.
1-65. ACCEPTABLE FINISHES. Any
varnish conforming to Federal Specification
TT-V-109, as amended, or other coating ap-
proved by the airframe manufacturer or the
FAA is acceptable. Exterior surfaces must be
further protected from the effects of abrasion,
weather, and sunlight. A number of systems
for exterior finishing have STC approval and
are manufactured under a PMA. (See Chap-
ter 2, Fabric Covering.) Low viscosity epoxy
adhesive (meeting the requirements of para-
graph 1-4 for that purpose) may be used as an
internal surface finish when subsequent bond-
ing is necessary.
1-66. PRECAUTIONS.
a. When making repairs, avoid exces-
sive contamination of surfaces with adhesive
squeeze-out at joints and on all surfaces. Ex-
cess adhesive should always be removed be-
fore applying finish. Because many paints and
adhesives are incompatible, even a slight
amount of adhesive underneath the finish may
cause premature deterioration of the finish.
b. Soiling substances, such as oil and
grease, should be removed as completely as
possible. Naphtha may be used to remove sur-
face deposits of oil and grease; however,
thinned residue may penetrate into any unpro-
tected wood. In areas where minor amounts of
oil or grease have penetrated the wood surface,
removal may be accomplished by use of an ab-
sorbent type of cleaner such as gunsmith's
whiting or a clothing spot lifter such as K2r™.
Marks that are made by grease pencils or lum-
ber crayons containing wax are harmful and
should be removed, but marks made by ordi-
nary soft graphite pencils and nonoily stamp
pad inks may be safely finished over. All dust,
dirt, and other solid particles should be
removed.
c. Sawdust, shavings, and chips must be
removed from enclosed spaces before they are
sealed off by replacement of skin. A vacuum
cleaner is useful for such cleaning.
d. Since most adhesives will not bond
satisfactorily to sealers, it is necessary to avoid
applying sealer over the areas where adhesive
will be applied. Mark off areas to receive ad-
hesive, and allow an additional 1/4 inch on
each side of the adhesive area to provide for
Par 1-64
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